The invention relates to a method of making hot strips of a workable lightweight construction steel which in particular can be easily deep-drawn cold.
The hotly contested automobile market forces the manufacturer to continuously look for solutions to lower the consumption of the fleet while retaining highest possible comfort. Weight saving plays hereby a crucial role. To address this desire, the suppliers, especially in the area of the body, use steel of higher strength, without adversely affecting buckling resistance as well as workability to deep-draw and/or stretch-form and the coating.
EP 0 889 144 A1 proposes a solution, using a cold-workable austenitic lightweight construction steel, which in particular can easily be deep-drawn and has a tensile strength of up to 1100 MPa. The main elements of this steel are Si, Al, and Mn in the range of 1 to 6% Si, 1 to 8% Al, and 10 to 30% Mn, the remainder being iron, including common incidental steel elements.
The high deformation degree is realized by TRIP (Transformation Induced Plasticity) and TWIP (Twinning Induced Plasticity) characteristics of the steel. Steels with high Mn content tend to segregate as experienced during conventional extrusion as a result of bending, bulging of the strand, sedimentation, and segregation by suction in the sump peak area.
The macrosegregation, obtained in this way and possibly resulting also in intermetallic phases, causes major strip defects during hot rolling.
In general, high-alloy steels also have a tendency for internal cracking, which ultimately also represent macrosegregation defects. They are caused, e.g., by bending stress during production.